Hammer driver controller for impact printers

Abstract

To obtain a substantially uniform print density while printing at high speed with a hammer-type impact printer, the hammer is electromagnetically driven in response to an energizing current which decays with a predetermined time constant from a high initial level toward a set point level which is automatically adjusted for different classes of characters so that the impact energy imparted to the hammer is regulated as a function of the character being printed. To adaptively absorb the shock forces generated by the rebounding hammer, the energizing current for the electromagnetic driver is reduced to a predetermined fraction of the set point level.

What is claimed is: 
     
       1. In an impact printer having a hammer for serially printing replicas of selected characters, belonging to different predetermined classes, on a recording medium in response to energizing current applied to an electromagnetic driver for said hammer, and   means for supplying a control signal to differentiate between said different classes of characters;   the improvement comprising a controller for supplying said energizing current for said hammer driver; said controller including   means for establishing different finite set point current levels to obtain a substantially uniform printing density for characters of said different classes,   means for selecting an appropriate one of said set point levels in response to said control signal as a prelude to printing each of said selected characters, and   means for causing said current to decay from a high initial level toward the selected set point level during the printing of each of said selected characters, whereby an impulse force is applied to said hammer to aid in overcoming any static friction and a controlled impact energy is imparted to said hammer to achieve substantially uniform density in the printing of each of said selected characters.   
     
     
       2. The improvement of claim 1 wherein said hammer is mounted for reciprocating movement toward and away from said recording medium to print on a forward stroke and rebound on a return stroke. 
     
     
       3. The improvement of claim 1 further including bias means coupled to said hammer for biasing said hammer rearwardly away from said recording medium; and wherein said driver comprises a pair of poles, a coil for generating a magnetic field between said poles in response to said energizing current, a rotatably mounted actuator arm having a free end in alignment with said hammer, and a pole piece mounted on said actuator arm in facing relationship with said poles, whereby said actuator arm is rotated to propel said hammer forwardly against said bias as said pole piece is attracted toward said poles in response to the energization of said coil. 
     
     
       4. The improvement of claim 1 wherein said hammer is mounted for reciprocating movement toward and away from said recording medium to print on a forward stroke and to rebound on a return stroke; and further including means for baising said hammer rearwardly away from said recording medium, and means for propelling said hammer forwardly against said bias in response to the energization of said driver.